There are numerous devices, applications and situations in which one needs to view an object through an intervening transparent medium. For example, most cell phones, computer displays, televisions and appliances employ displays that include a top transparent sheet as the transparent medium through which the displayed information or picture is viewed. Likewise, windows, windshields, glass for covering photographs and other artwork, aquariums and the like involve viewing an object through an intervening transparent medium.
A common problem that arises when viewing an object through an intervening transparent medium is glare. Glare may be defined as the substantially specular reflection of ambient light on the viewer side of the transparent medium from one or more surfaces of the transparent medium. Thus, glare light travels an optical path that extends from the source of the ambient light to the surface of the transparent medium and then to the viewer, with the angle of incidence being substantially the same as the angle of reflection. Object light, on the other hand, travels from the object through the transparent medium to the viewer. Glare makes it difficult to view an object through the intervening transparent medium when the optical paths of the glare light and the object light substantially overlap in the region between the transparent medium and the viewer.
Consequently, anti-glare surfaces are often applied to the viewer-side surface of the transparent medium to avoid or reduce the amount of glare. Such anti-glare surfaces are typically formed by providing some degree of roughness that spreads (i.e., scatters or diffuses) the light reflected by the surface over a certain angle. Typical anti-glare surfaces used in display applications comprise a coated or structured polymeric film (often a polarizing film) that is directly laminated to the surface of the front glass sheet forming the display (e.g., a liquid-crystal display (LCD)). The ideal parameters and processes used for anti-glare polymeric coatings are not necessarily the same as the ideal parameters used for a protective anti-glare cover glass. One reason for this is the anti-glare surface on a protective cover glass typically must be placed at a larger optical distance from the image-forming plane of the display device than would an anti-glare polymeric coating.
Random noise may be generated in an image viewed through such an anti-glare surface due to either excessive roughness or the shape of the features that form the roughened surface. Such noise is generally called “sparkle” or “dazzle” and may be characterized by a number referred to as the pixel power deviation (PPD). Sparkle may occur when anti-glare or light-scattering surfaces are employed on the surface of a transparent medium. Sparkle is associated with a very fine, grainy appearance that appears to shift as the viewing angle changes. This type of sparkle is observed, for example, when pixelated displays such as LCDs are viewed through an anti-glare surface. “Sparkle,” as the term is used herein, is of a different type and origin than “speckle,” which is an interference effect that arises in connection with rough surfaces illuminated by coherent light.
A major shortcoming of anti-glare and anti-sparkle surfaces is when applied to an intervening transparent medium disposed between the user and the object, they distort the optical path of the transmitted light. For example, conventional anti-glare and anti-sparkle surfaces relying on surface roughness tend to diffuse the object light, which makes the object look diffuse and thus less clear. The farther the object is located from the transparent medium, the more distorted the object appears when viewed through the transparent medium. Thus, there is a need for anti-glare and anti-sparkle surfaces having reduced optical distortion for object light when applied to a transparent medium.
A related application for embodiments of the present disclosure is the use of roughened surfaces on touch screens or other touch-sensitive surfaces through which light is transmitted. These may often be used to improve the “gliding feel” of fingers, styluses, or other probes over a touch screen surface. This may be accomplished through adding surface roughness, which reduces the effective contact area between probe and screen, thus reducing the effective friction or stick-slip effects, and providing a pleasing touch interface. These rough surfaces, while not used strictly to create an anti-glare effect, typically also will create the same problems as described above, such as distortion or sparkle effects for transmitted light. Thus, a related aspect of this disclosure is to create roughened surfaces to enhance touch screen or touch-sensitive-surface usability through reducing effective friction or creating an engineered friction surface, while at the same time minimizing negative optical effects imparted to the transmitted light.